The present invention relates to a high intensity discharge lamp that produces a radiation spectrum suitable for various applications, such as image projection, automotive, medical, communications (optical fibers) and general lighting applications.
Image projection is one of the major fields of application for visible light generated by a high intensity discharge (xe2x80x9cHIDxe2x80x9d) lamp. The conventional HID lamp optimized for visible light has major attributes that render it particularly suitable for use in image projection. Such HID lamp typically emits light from a plasma arc formed inside an envelope between two electrodes which are spaced a particular distance apart. The radiation spectrum of the light emitted from the HID lamp depends on the gases and other materials contained within the lamp (the xe2x80x9cfillxe2x80x9d). In a conventional projection system, the light from the lamp is collected via a series of optical elements and projected through an image gate onto a screen to form a projected image. The element which forms the image at the image gate can be film or any type of a light modulator, e.g., liquid crystal displays (xe2x80x9cLCDxe2x80x9d), digital micro-mirror devices (xe2x80x9cDMDxe2x80x9d) or liquid crystal on silicon displays (xe2x80x9cLCoSxe2x80x9d). In image projection applications, the utility of the HID lamp may be defined by its optical efficiency, power efficiency, color rendition, arc stability (absence of xe2x80x9cflickerxe2x80x9d), arc gap, physical size, initial cost, operating cost, and overall system cost. HID lamps can also be designed to produce ultraviolet (xe2x80x9cUVxe2x80x9d) or infra-red (xe2x80x9cIRxe2x80x9d) radiation for applications with similar performance requirements.
A conventional HID lamp presently has light transmissive envelopes made from quartz or polycrystalline alumina (xe2x80x9cPCAxe2x80x9d, also known as xe2x80x9cceramicxe2x80x9d envelopes). In general, image projection applications require the HID lamp with a clear envelope, small arc sizes and narrow light beams. The HID lamp with quartz envelopes generally meets these requirements, however, PCA envelopes are translucent and generally not suitable for image projection and similar applications. The PCA envelope lamp is usually constructed with relatively large gaps as necessary for large light source applications. More recently, the HID lamp envelope has been made from poly-crystalline sapphire (xe2x80x9cPCSxe2x80x9d) which is produced by conversion in place of PCA envelopes. Although PCS envelopes improve light transmissivity and other characteristics of the envelope compared to PCA envelopes, PCS envelopes still have microscopic surface undulations that render them not suitable for most image display projection and related applications. Therefore, the conventional HID lamp continues to rely primarily on quartz envelopes.
The use of a quartz envelope places substantial limits on the conventional HID lamp in terms of meeting the above listed desired features for image projection. For example, the quartz envelope has a relatively low melting temperature, power load factor, thermal conductivity and tensile strength. Such considerations effect the lamp optical efficiency, efficacy, power capacity, size, life and the ability to control flicker. Furthermore, the quartz envelope is permeable to a number of additives, such as sodium or hydrogen, which are important in the spectral tailoring of the emitted light.
The Image Projection Industry has established that a correlated color temperature (xe2x80x9cCCTxe2x80x9d) of 6,500xc2x0 K (xe2x80x9cDxcex3standardxe2x80x9d) is the light source spectrum most desirable for image projection because it has a high color rendition index and is close to daylight quality. The conventional quartz envelope HID lamp is generally designed to operate at pressures from about 120 up to a maximum around 200 atmospheres utilizing a fill of pure mercury. However, a high pressure mercury lamp has CCT about 7,000xc2x0 K to 9,000xc2x0 K. The light from such HID lamp must be filtered in order to achieve a more compatible CCT however filtering can reduce lamp efficiency by about 30 to 40%. Metal halide additives have typically been added to mercury lamps for the purpose of tailoring the light spectrum to a more desirable CCT (xe2x80x9cmetal halidexe2x80x9d lamps). However, the effectiveness of metal halides is reduced as operating pressure increases to the point of minimal contribution at the maximum current operating pressures for the quartz envelope lamp. A conventional Image projection system uses light sources with a wide range of CCT from a typical 3,000xc2x0 to 3,300xc2x0 K tungsten halogen lamps, to 4,000xc2x0 to 5,000xc2x0 K for metal halide HID lamps, 5,500xc2x0 to 6,500xc2x0 K for short arc Xenon lamps, and over 7,000xc2x0 K for a mercury lamp.
In the image projection field, the industry has moved steadily in recent years toward utilizing smaller light modulators based upon foundry fabricated silicon wafers, e.g., DMD and LCoS, with diagonals of 0.9 down to 0.5 inches. Such small apertures require that the HID lamp used have arc gaps in the range between 0.8 mm-1.3 mm in order to obtain an efficient optical match between the light emitted by the HID lamp and the aperture optics. As lamp gaps become smaller the efficacy of the HID lamp is reduced and the power that can be supplied to the plasma arc is limited by the envelope material thermal characteristics. In order to increase the efficacy of smaller arc gap lamps, the operating pressure must be increased. However, quartz envelope properties limit the pressure and power load factor that one can use in such HID lamps to about 200 atm and about 20 watts/cm2. Also, in applications such as image projection, lamps must be essentially flicker free. Flicker in an arc lamp is associated parametrically to the lamp bulb size and the fill pressure. Using conventional quartz envelopes, one needs to remain below 200 atm in lamp pressure in order to achieve flicker free operation.
The object of the present invention is to improve the efficacy, lifetime and spectral stability of a high intensity discharge (xe2x80x9cHIDxe2x80x9d) lamp. The present invention utilizes single crystal sapphire (xe2x80x9cSCSxe2x80x9d) in an envelope of the lamp to replace conventional envelope materials. The SCS envelope lamp according to the present invention may be physically smaller, generate light more efficiently, and produce a plasma with greater luminance and stability than a conventional HID lamp. The SCS envelope lamp may be utilized, e.g., in applications that require a small, powerful light source with a narrow beam width such as image projection, automobile headlamps, fiber optic light sources, and the like.
SCS has substantially superior properties compared to conventional materials (e.g., quartz or polycrystalline alumina) that are utilized in the envelopes of the conventional HID lamp. These properties include higher tensile strength, greater burst pressure resistance, higher softening and melting points, greater thermal conductivity, and a higher power load factor. These advantages allow the SCS envelope lamp according to the present invention to operate at higher pressures and temperatures and produce more usable light per watt of power input. In addition, the superior chemical resistance of SCS permits the use of a broader range of fill gases and additives to produce light in a specific spectrum for the application. For example, for visible light radiation in the 400 nm to 700 nm spectrum, this versatility should allow correlated color temperatures to be set and consistently held in a narrow range between 4,000xc2x0 K to 9,000xc2x0 K. In addition to visible light radiation, the present invention may also be utilized to produce radiation emissions in the ultraviolet (200-400 nm) and near infra-red (700 nm to about 2,500 nm) spectra with similar benefits.
The SCS envelope lamp may have an effective life four to five times longer than a conventional quartz envelope lamp, even when operating at significantly higher temperatures and pressures. This is accomplished by matching the thermal expansion characteristics of the seal materials and other components to those of the envelope, thereby minimizing the stress on the seals. In addition, the SCS envelope lamp may be manufactured to tighter tolerances with greater consistency than quartz or polycrystalline alumina, and, by using automated manufacturing techniques, at the same or lower cost.
The plasma in the SCS envelope lamp may be produced in a continuous non-flash mode by providing a constant voltage across two end electrodes in waveforms suitable for high pressure operations. The SCS envelope lamp may utilize direct or alternating current. In another embodiment, the SCS envelope lamp may be without electrodes and powered by microwaves or radio frequency radiation. Alternatively, the SCS envelope lamp may be operated as a hybrid using both electrodes and microwave power.